Energy saving is globally seen as vital for our environment. Legislation in North America and Europe is requiring energy saving measures.
Within the field of building automation, building automation equipment controlled by sensors is an important enabler for achieving the energy saving. In particular in the field of lighting systems, occupancy sensors and light sensors used for daylight harvesting are widely used for energy saving. Daylight harvesting uses a light sensor to control the light output of an artificial light dependent on the ambient light or daylight such that a light level in a room can be kept constant. When, during daytime, sunlight enters the room, and contributes to the light level in the room, the light output of the artificial light may be reduced with increasing ambient light, whereby energy is saved.
Although a sensor may be coupled to a building automation controller of building automation equipment by wires, flexibility of installation and reduction of installation costs for fitting and for retrofitting may be obtained by using wireless sensors, as they require no (new) wires. At least the low installation costs justify any extra costs of the wireless sensors. For a truly wireless sensor, not only the communication between the sensor and the building automation equipment or the communication between different sensors is wireless, but also the power supply of the sensor is wireless. This can be achieved by using a battery or by energy harvesting/scavenging, such as by a photovoltaic solar panel.
A wireless sensor comprises at least a sensor element for measuring a quantity level, a microcontroller device at least for obtaining and processing a measuring value from the sensor element, and a communication device for transmitting a data signal.
A wireless sensor provides a measuring signal to a controller of the building automation equipment by a suitable wireless communication transmission, e.g. based on the ZigBee standard (IEEE 802.15.4). The communication of a wireless sensor consumes a lot of power, so a communication duty cycle should be kept as low as possible. Also the sensor should be put in a sleep mode when not used. According to the ZigBee standard, this is supported via so-called “sleeping end devices” in a ZigBee network.
For a wireless light sensor that monitors a light level, the power consumption may be 40 μA on average in an active mode of the microcontroller, based on a CC2430 device manufactured by Chipcon/Texas Instruments. A microcontroller of the light sensor samples the sensor element every second to measure a light level, and to provide a corresponding wireless measuring signal via the light sensor's communication device to a light system controller to dim an artificial light to a required level. However, in most building applications, such as office applications, artificial light is not necessary during a complete 24 hour day. Consequently, the light level in the room need not be monitored 24 hours a day through light sensors. If the light level e.g. needs only be measured during 8 hours a day for 5 days per week, the average power consumption will be around 10 μA. Taking a 1000 mAh battery as a reference, the battery lifetime may then be more than 11 years.
Although such an operation of the light sensor is advantageous in saving energy and prolonging battery lifetime, when the (microcontroller of the) light sensor is in a sleeping mode, the communication device is active with a very low duty cycle, and thus is most of the time turned off When the communication device component is turned off, the photosensor is unable to communicate with other devices. The light sensor only receives messages shortly after itself has sent a message.
This blocks the possibility of the light sensor to retrieve a lamp status, as communication is impossible when the (microcontroller of the) light sensor is in a sleeping mode. If one would wake up the light sensor every minute to check the light level, this would still cost considerable energy, e.g. an average current of 8 μA. In case of a light level check every minute, an average latency in a light control loop of the lighting system controller would be 30 seconds, which is considered unacceptable. Also the actual, current light level would be unknown for the light control loop. Consequently, the light control loop would start at a fixed point, and change the dimming of the artificial light from the fixed point to a correct light level. This is undesirable.